An integrated adsorption/cryogenic distillation process for the separation of an air feed wherein (1) the air feed is passed through a vacuum swing adsorption (VSA) unit to remove impurities comprising water and carbon dioxide which will freeze out at cryogenic temperatures, (2) the VSA sequence includes an adsorbent regeneration step whereby the impurity-saturated adsorbent is purged under vacuum with a purge gas, (3) the resulting impurity-depleted air feed is fed to a cryogenic distillation column system for rectification into a gaseous nitrogen overhead and a liquid oxygen bottoms and (4) a waste stream from the distillation column system is warmed against the impurity-depleted air feed to recover its refrigeration and subsequently recycled as purge gas for the VSA unit is taught in the art. Specifically, U.S. Pat. No. 5,156,657 by Jain et al. teaches such a process. Because the adsorption portion of the process uses a vacuum purge step instead of the conventional ambient pressure purge step, a key advantage of the process is that the purge gas requirement is significantly reduced or, at the same purge gas consumption, the adsorbent bed size is significantly reduced. This is important when only a small quantity of waste from the cryogenic distillation portion of the process is available for purging purposes.
There is a concern with Jain, however, in that Jain does not recover all of the available refrigeration from the cryogenic waste stream prior to recycling such waste stream as the purge gas for the VSA unit. This concern stems from the positioning of the expansion of Jain's cryogenic waste stream to the sub-ambient pressure level which expansion is required before such waste stream can be used as the purge gas for the VSA unit. Specifically, such expansion in Jain is performed after the waste stream is warmed against the air feed to the cryogenic portion of the process. As a result, the refrigeration that is obtained upon expansion of the waste stream is not recovered in the cryogenic portion of the process but instead is largely wasted in the adsorption portion of the process. It is an object of the present invention to address this concern by performing the expansion of the cryogenic waste stream before the waste stream is warmed against the air feed to the cryogenic portion of the process.